What is gradient as it relates to graphic design software?

Graphic design often uses the term ';gradient';; what exactly does that mean? Can you point me to a good example with explanation?What is gradient as it relates to graphic design software?
Definition: A gradient, or graduated fill, is a color fill that gradually blends from one color to another. A gradient can contain more than two colors, all fading smoothly from one to another. Gradients are very useful for creating shading for three-dimensional and metallic effects. Most graphics software allows you to create a variety of gradient effects such as linear, radial, reflected, diamond, conical, and angle. Some software even offers gradient mesh effects, which allows you to create elaborate shading by plotting colors to each of the intersections of an imaginary mesh, so that the colors all blend together. In the example image you can see several types of gradients, including a 3D button created simply by using two opposing linear gradient fills.